The invention relates to a method for charging at least one capacitive control element, in particular a fuel injection valve of an internal combustion engine. The invention also relates to an apparatus for carrying out the method.
One of the advantages of actuating fuel injection valves of an internal combustion engine through the use of piezoelectric control elements instead of solenoids is the short switching time of the control elements. That leads to steep and very abrupt rates of change and only small variations in the amounts of fuel being injected. Charging times that are as short as possible are desired from the combustion design point of view.
In order to achieve a smoother combustion profile, the amount of fuel is split into an initial and a main injection amount, which allows slower combustion and thus a reduction in combustion noise. Until now, the control elements have been actuated with a constant charging and discharging time (duration of the charge reversal from an energy source to the control element or vice versa), which must be very short (for example 100 xcexcs), to ensure that a predetermined initial in-jected amount of fuel can still be injected even in the upper-most load or speed range of the internal combustion engine.
The charging process is carried out, for example, as a ringing process with the charge from a charge source (a series circuit including a charge capacitor and a charge reversal capacitor) through a charge reversal coil to the control element. The inductance of the charge reversal coil together with the capacitances of the capacitors and of the control element determine the time constant for the charging and discharging processes (the charging and discharging times). Such an apparatus is disclosed in German Patent DE 196 52 801 C1, corresponding to U.S. Pat. No. 6,121,715.
However, the short charging times lead to high noise emissions in frequency bands which are unpleasant for human hearing. For example, in a motor vehicle, it is found to be very disturbing if the combustion noise is low when the internal combustion engine is idling.
It is accordingly an object of the invention to provide a method for actuating a capacitive control element of a fuel injection valve of an internal combustion engine that allows a considerable reduction in control element noise emission and an apparatus for carrying out the method that requires little component complexity, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and apparatuses of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for charging at least one capacitive control element, in particular a fuel injection valve of an internal combustion engine, which comprises providing a tuned circuit having a capacitive charge source, a charge reversal coil and the at least one control element; dimensioning a capacitance of the charge source for a predetermined maximum charging time; and achieving a shorter charging time by prematurely disconnecting the tuned circuit at a specific time after a start of a charging process and switching over the tuned circuit in a freewheeling circuit having the charge reversal coil and the control element.
With the objects of the invention in view, there is also provided an apparatus for charging at least one capacitive control element, in particular a fuel injection valve of an internal combustion engine, comprising a series circuit including a charge source having a charge capacitor to be charged from an energy source and a charge reversal capacitor, a charging switch, a first diode connected to the charge reversal capacitor at a first junction point, a charge reversal coil connected to the charge reversal capacitor at a second junction point, and the at least one control element; a discharging switch connected to a reference ground potential; a second diode connected between the first junction point and the discharging switch and forward-biased toward the reference-ground potential; a third diode connected parallel to the charge reversal capacitor and forward biased toward the at least one control element; and a fourth diode connected between the reference ground potential and the second junction point and forward biased from the reference-ground potential toward the charge reversal coil.
Through the use of such an apparatus, the charging and discharging times of a capacitive control element, particularly in the low-load and idling range of the internal combustion engine, are varied by various measures during the charging process, for example in a range between 100 xcexcs and 200 xcexcs.
The achievement of the method according to the invention is that, in order to obtain shorter charging times (and thus smaller injected amounts), the current flowing during the charging process of the capacitive control element in the tuned circuit including capacitors, a charge reversal coil and a capacitive control element, is diverted into a freewheeling circuit, through the use of which the charging process is considerably shortened.
With regard to the choice of optimum charging times it can be said that: the duration of the charging time limits the minimum fuel injection duration. This is critical in particular with high injection pressures, since the injected amount of fuel rises with the fuel pressure, in proportion to the load, for the same injection duration. Ever shorter injection durations are thus required as the fuel pressure rises, in order to achieve a specific injected amount, in particular a small initial injected amount.
On the other hand, the injected amounts for the main injection are load-dependent and pressure-dependent. When the load is low, small injected amounts are required, but when the load is high, large injected amounts are required when the fuel pressure is high. This correlation between the amount of fuel and the fuel pressure allows longer charging times to be used for the main injection, in the high-load range as well.
Within certain limits, for example between 100 xcexcs and 200 xcexcs, and except for dead-time effects (delays at the start and end of injection) which can be compensated for by shifting the timing of the actuation signals, different charging times for a capacitive control element have no influence on the injection profile that is relevant for a combustion process.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and an apparatus for charging at least one capacitive control element, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.